Add support for launching activities when attaching to a car or desk dock.

Categories CATEGORY_CAR_DOCK and CATEGORY_DESK_DOCK can be assigned to
activities to make them launchable on docked events.
This is a better mechanism than listening for ACTION_DOCK_EVENT with a broadcast receiver.

Change-Id: Ic5f3ab3555ce02ca922bc31ebba41978cefe8bda
Signed-off-by: Mike Lockwood <lockwood@android.com>